My advice would be to pick up 10th anniversary rings asap for leveling.
Then go Novi grounds -> Spores -> Metalings -> Moorings -> Snow Harpies / Whatever you can get a party for.
Moorings can actually be faster than snow harpies these days for starting out with terrible gear as they are easier, and faster to down without a party.
Some people prefer Neko island but I still think they are crazy when you have a mob that gives you love taps, heals you, is a free warp if you save in poring city, is easy to get to unlock, and gives you nearly 40k exp each for only 4k hp? That ratio is beautiful.
Obviously the advice of, bring friends and party as much as possible still applies as you get the 20% boost per person, in addition to the exp boost for each person who hits the thing. Note that pets also count for the hitting stuff boost.
